Glycolic acid (also known as 2-hydroxyacetic acid or α-hydroxyacetic acid) can be used to make ethylene glycol. Glycolic acid can be produced by the reaction of aqueous formaldehyde and carbon monoxide in the presence of an acid catalyst. This reaction is often referred to as the “hydrocarboxylation” or “carbonylation” of formaldehyde. The formaldehyde reactant is generally prepared by well-known methods as an aqueous mixture that contains 35 to 70 weight percent formaldehyde. Typically the glycolic acid is first esterified with an alcohol, diol, or other polyol to produce glycolate ester oligomers which are then hydrogenated to produce ethylene glycol. The esterification step leads to faster reaction rates and better selectivity to ethylene glycol at milder operating conditions of temperature and pressure than applying hydrogenation of glycolic acid directly.
Esterifying glycolic acid and ethylene glycol produces a mixture of glycolate ester oligomers and glycolic acid oligomers. One of the products of these many reactions is water. The presence of water is undesirable for at least two reasons. The esterification reactions are equilibrium limited, so the removal of water favors the formation of glycolate ester oligomers. Secondly, water serves as a deactivator to the hydrogenation reaction catalyst in the process for making ethylene glycol. The hydrogenation catalyst deactivation not only is affected by water in the feed, but also water produced in the hydrogenation reactor. Each mole of glycolic acid and glycolic acid oligomers (i.e., each mole of acid-ends) in the feed produces one mole of water upon hydrogenation. There is a need in the industry to esterify glycolic acid such that the product stream has a limited amount of total moles of water plus acid-ends. There is also a need to produce glycolate ester oligomers for hydrogenation in a manner that is more economical in the overall process for producing ethylene glycol from glycolic acid.